In Assisted GPS mobile station positioning schemes, one or more ground based referenced station nodes coupled to a cellular communication network receive GPS satellites data and re-transmit the data in the form of assistance messages at higher data rate over the cellular air interface to mobile stations for use in position determination.
There are several types of GPS assistance messages. Ephemeris assistance messages include GPS ephemeris and clock correction data. Almanac assistance messages include almanac and other data, which includes generally truncated ephemeris, ionospheric delay elements, universal time coordinate (UTS) offset, and other data. Differential GPS (DGPS) assistance messages include differential correction data.
In some cellular communications architectures, for example the 3rd generation (W-CDMA/UMTS) architecture, one or more a value tags are included in a header, or Master Information Block (MIB) transmitted separately from the assistance message by the network. In the W-CDMA/UMTS architecture, the assistance message is referred to as a System Information Block (SIB). The MIB and SIB are generally transmitted according to different schedules.
The value tags of the MIB generally provide scheduling information and inform the cellular mobile stations whether the content of the corresponding assistance message has been updated. Currently, a multi-bit cell value tag is used for Global Positioning System (GPS) assistance messages in the W-CDMA/UMTS specification. The GPS value tag is updated whenever any data, for example the GPS time stamp (“Transmission TOW” in the W-CDMA/UMTS specification), of the assistance message (SIB in W-CDMA/UMTS) is updated or changed.
Each assistance message, or SIB, is valid generally for a specific geographic area, for example for a particular cell or for a particular Public Mobile Land Network (PLMN). When a new MIB is read by the mobile station, its value tag for the corresponding SIB in the local cell or PLMN is compared to the value tag of the corresponding SIB stored previously in the mobile station. Presently, the mobile station (“User Equipment” in the W-CDMA/UMTS specification) updates the SIB any time the value tag of the corresponding MIB has been updated. Reading the SIB however consumes substantial power, which is a limited resource in battery powered mobile stations.